1. Field of the Invention
This invention relates to a method and apparatus for characterizing composite materials, and in particular, to utilizing an artificial neural network for predicting an impact resistance of a composite material.
2. Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Composite materials have been in human use in different forms for thousands of years, examples of earlier use of composite materials may be seen in the mud and straw bricks.
Composite materials for construction, engineering and other similar applications are formed by combination of two or more materials in order to enjoy the benefits of the properties of the constituents. A property of composite materials is that the materials are still distinguishable and don't blend completely unlike alloys, hence, normally exhibit an interface between one another. The constituent materials retain their physical and chemical properties, only to combine to give properties that are not offered by the individual constituents.
The majority of composite materials use two constituents: a binder or matrix and reinforcement. The reinforcement is stronger and stiffer, forming a sort of backbone, while the matrix keeps the reinforcement in a set place. The binder also protects the reinforcement, which may be brittle or breakable.
As illustrated in FIG. 1, composites may be categorized in three main divisions according to the geometry of the reinforcements: (1) Particle-reinforced, (2) Fiber-reinforced, and (3) Structural Composites.
According to the type of the matrix, there are: (1) Polymer Matrix Composites, (2) Metal Matrix Composites, and (3) Ceramic Matrix Composites.
Technologically, important composites may be those in which the dispersed phase is in the form of a fiber. Design goals of fiber-reinforced composites often include high strength and/or stiffness on a weight basis. In fiber-reinforced composites, fibers are the phase that provides the strength and the ability to carry load while the matrix increases the ductility and also acts as binding agent for the fibers and also acts as load transfer medium.
Common fiber reinforcing agents include, Aluminum, Aluminum oxide, Aluminum silica, Asbestos, Beryllium, Beryllium carbide, Beryllium oxide, Carbon (Graphite), Glass (E-glass, S-glass, D-glass), Molybdenum, Polyamide (Aromatic polyamide, Aramid), e.g., Kevlar 29 and Kevlar 49, Polyester, Quartz (Fused silica), Steel, Tantalum, Titanium, Tungsten, Tungsten monocarbide.
Common resin materials include Epoxy, Phenolic, Polyester, Polyurethene, and Vinyl Ester.
Composite pipes are gradually replacing the conventional pipes in the industrial applications. Composite pipes show good resistance to corrosion compared to metallic pipes in applications where pipes are carrying fluids like water or highly corrosive sulphuric acid is present in it. This property makes them ideal for usage in pipe industry [37].
Composite pipes may be described in two categories depending upon the type of resin material: (1) Reinforced thermosetting resin pipes (RTRP), and (2) Reinforced thermoplastic pipes (RTP).
Due to their superior mechanical and thermal properties over conventional materials, fiber reinforced composite materials are preferred in the petroleum industry. As an example of the advantage gained by replacing conventional material pipelines with composite materials is that a 6-inch diameter pipe weighs 4 pound per foot, whereas copper nickel pipe with the same diameter weighs 24 pound per foot [51].
Another major area of significant interest where composite pipes may be of use is the water related applications. Lack of fresh water reservoirs put forward the need of desalination applications. The desalination application requires piping systems that are corrosion resistant [61]. Water losses due to degradation of traditional pipe systems present a significant financial and maintenance problem. Composite based piping systems provide good protection against the corrosion. Fiberglass pipe systems have become the material of choice in the desalination and water distribution industry.
There are several major advantages composite pipes offer over conventional material pipes, such as corrosion resistance. Fiberglass pipes are resistant to corrosion for a long period of time and resists corrosion to a variety of media including seawater, hot brine, acids and other chemicals [61]. Also, the composite materials have a high strength to weight ratio compared to metals and the transportation and installation of the composite materials is easier. Large lengths of composite pipes may be easily manufactured and may be assembled with relative ease on sites.
Since, composite materials are corrosion resistant; the cost of maintenance is considerably lower. Also, the fatigue resistant capability of composite pipes is better than the metallic pipes. Also, low internal friction, fire resistance, torsional stiffness and good impact resistance combined with the flexibility in design as per strength and other requirements make them ideal replacement for the current conventional materials [61].
Mechanical damages to pipes occur frequently. These damages may cause leakage of oil and gas from pipes resulting from structural failure and may lead to reduced operating pressure or stopped production, human and environmental hazards and the heavy economic losses [7].
There are, however, some issues related to the use of composite piping systems primarily the lack of test data to support the materials' long term durability. The failure caused by the mechanical damages is one of the important aspects that need to be addressed. The structural failure of these pipelines may be due to a number of effects as burst, impact, puncture, overload, buckling, fatigue and fracture.
One of the major causes of damages in pipes are considered as “External Damage” caused by foreign objects and third party damage such as caused by a farmer ploughing a drainage ditch, or a supply boat dragging its anchor around an offshore platform [24]. These structural components are often very susceptible to foreign object impact during service. These damages may be vulnerable and may go unseen especially in case of low velocity impacts since these are not visually observable. A small dent caused by such impacts may lead to significant underlying damages for example, delamination, matrix cracking, fiber breakage and fiber/matrix interfacial debonding induced within the laminate [27].
Outside forces are one of the major causes of pipeline failures. Historically, the pipelines used were made from steels. Steel is a ductile material and the specifications used in the industry are already set for its use. The ASME codes B3 1.4 for oil applications and B31.8 for gas applications provide measures for the different kind of damages and repairs [12]. These materials are tested for their ductile behavior. Impact tests are considered good method to measure toughness of pipelines.
During the product lifecycle it is always expected that damages may occur due to impact by foreign objects. Mechanical damage may occur during handling, installation and service to the composite pipes. To ensure the reliability, good impact properties against low and intermediate velocity impacts are needed. Due to the laminate structure of composite materials their behavior to impacts is different to the metallic structures. The modes of damage in composite structures due to impact may be categorized as matrix cracking, fiber breakage and/or delamination [14].
Impact generally causes low to medium energies which cause a global structural response, and often results in internal cracking and delamination, while at higher energy levels may cause penetration and excessive local shear damage [1].
The impact damage may be caused by a number of factors, some of which are for example:
Dropped tool
Damage due to mishandling
In-service impacts
Hail and debris
The composite materials are prone to low energy impacts that may be observed with the effect of delamination in the plies and may be indirectly responsible for the failure. Delamination result in lowering of the elastic moduli, strength, durability and damage tolerance [14]. Low velocity impacts may also cause matrix cracking which sometimes may not be on the surface of impact but on the internal or bottom surface, this is due to the fact that the laminate is flexible. Matrix cracking is in the perpendicular direction to the plane of the laminate and is a tensile crack. In thicker laminates, matrix cracking is near the top surface and characterized as the shear crack.
The damage in composite materials due to impact force is a complex mechanism and still there are no analytical methods that may be generally accepted to define the phenomenon.
In addition to these, the micro failure modes commonly observed in composite laminates are fiber breakage, fiber micro buckling and matrix crushing, transverse matrix cracking, transverse matrix crushing, debonding at the fiber-matrix interface and delamination [14].